An insulated gate bipolar transistor (IGBT) has a high input impedance of a field effect transistor and a high current drive capability of a bipolar transistor, and is particularly suitably used as a power switching device.
Conventionally, an IGBT comprises an N− type base region, a P type base region formed in a predetermined surface region of the N− type base region, an N+ type emitter region formed in a predetermined surface region of the P type base region, a P+ type collector region formed at the lower surface of the N− type base region via an N+ type buffer region, a collector electrode electrically connected to the P+ type collector region, a gate insulating film formed on a predetermined surface region of the N− type base region, a gate electrode formed on the gate insulating film, and an emitter electrode electrically connected to the N+ type emitter region.
In an IGBT structured in this manner, since the P+ type collector region is formed at the lower surface of the N− type base region via the N+ type buffer region, carriers are stored in the N+ type buffer region or in the N− type base region near the N+ type buffer region when the device is turned off. Since there is no path for the stored carriers to be discharged, a tail current continues flowing until they disappear by their recombination, resulting in a problem that the speed to turn off becomes slow.
Though there is a method of introducing a lifetime killer for promoting the carrier recombination to increase the speed to turn off, this in turn causes a problem that the forward-direction voltage is increased.
Hence, an IGBT has been developed, in which an N+ type collector-short region is formed in the P+ type collector region so that the carriers in the N+ type buffer region or in the N− type base region near this region are discharged immediately (for example, patent Literature 1).
Since the IGBT disclosed in the patent Literature 1 can discharge the carriers stored in the N+ type buffer region or in the N− type base region near this region through the N+ type collector-short region when it is to be turned off, it can increase the turn-off speed. Further, since it does not use a lifetime killer, its forward-direction voltage characteristic is not damaged.
Patent Literature 1: Unexamined Japanese Patent Application KOKAI Publication No. H5-3205